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• For a long time the measurement of a healthy baby was having 10 fingers and 10 toes. Wait,

• why didn't anybody care about the eyes, nose, or anything else?

• Hey digitals, Lissette here for Dnews. I am the proud owner of 10 fingers and 10 toes,

• I can prove half of this just right now. Youll just have to trust me on the 10 toes thing. But,

• have you ever wondered why that is? Why don’t we have 12 fingers and 14 toes? Or 6 fingers

• and 1 toe? Well, this may not come as a shocker to you DNews fans out there, but no one really

• knows. It might all come down to math.

• Mark Changizi, a theoretical neurobiologist, developed a hypothesis called the Limb Law,

• first published in the Journal of Theoretical Biology in 2001. It’s a mathematical equation

• that tries to figure out the correct number of limbs an animal needs in order to connect

• to the world around it based on the length of its limbs relative to it’s body size.

• Okay, wow. The equation looks like this. Confusing, I know. But he used this equation with 190

• different animal species from seven different animal Phyla, and the math seems to check

• out. Mark thinks this same equation could be used to figure out why humans have five

• fingers on each hand.

• The calculation treats our palms as the animal and our fingers as limbs. Also it takes into

• account that the whole reason behind having fingers is that theyre able to fold over

• our entire palm, in order to grasp things. So, first, fingers have to be at least as

• long as our palms, also known as length need to connect to the outside world. When you crunch the

• numbers, you'll get 9.42 digits per hand. But, he argues, since our hand is not a free

• roaming creature, but is attached to our arm, it doesn't need limbs protruding from all

• sides. Hands only need half the total number of limbs. That gets us to 4.71 fingers per

• hand. Just based on math. Pretty amazing.

• But other explanations are simpler than that. Evolutionary biologists say that we have 10

• fingers and 10 toes because that’s the best combination for us. If 8 fingers and 8 toes

• would have given us an evolutionary advantage, humans would have 4 fingers on each hand.

• Cliff Tabin, a Harvard Medical school geneticist focusing on limb development in vertebrates,

• pointed out that our motor skills come down to our thumb and index finger, and that another

• small, 6th finger wouldn’t really change anything. And if we ever did grow a 6th finger,

• Tabin argues, it would probably grow out of our wrists, like panda’s . It gives

• them extra support when gripping bamboo, which gives them an evolutionarily advantage. So,

• basically, we'd have six if people with six fingers got an advantage.

• Then, there is this more complicated science answer to why we have the number of digits

• we do. James Sharpe from the Centre for Genomic Regulation in Barcelona discovered

• it all comes down to three molecules in our body. Bmp, Wnt and Sox9. Theyre responsible

• for marking out in an embryo where fingers will go and how far apart theyll grow.

• And for the most part it marks out space for five fingers on each hand and five toes on

• each foot. But of course sometimes things can go wrong, resulting in the gaps between

• fingers being bigger and only having room for four fingers. Or, sometimes it screws

• up and you have room for an extra finger. Oh and fun fact, the basis for this discovery

• was a theory first proposed by Alan Turing back in 1952 about how animals get their spots and

• stripes. Love you, Alan.

• So why 10 fingers and 10 toes? Again, were still not entirely sure. But it all seems

• to come down to because that’s exactly the number of fingers and toes we need. Sometimes

• it’s best to just trust evolution and be thankful we don’t have hoofs. Amirite?

• So that’s hands. But what about feet?

• When did they evolve and why do they look

• different from hands? Julia walked a mile in those shoes, here.

• Toes likely evolved from the same bones that became fins, wings and hoofs in other species.

• The dominating scientific theories are that fingers and toes are exclusive features of tetrapods.

• Or vertebrates with limbs.

• But according to one study toes may evolve even before our fishy ancestors left the ocean.

• Why do you think we have 10 fingers and toes? Would you rather have more or less?

• Let us know on the comments and remember to subscribe to DNews for more episodes everyday.

• Thanks for watching, guys.

For a long time the measurement of a healthy baby was having 10 fingers and 10 toes. Wait,

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# なぜ指と足の指が10本もあるのか？ (Why Do We Have 10 Fingers and 10 Toes?)

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Angela Hsu に公開 2021 年 01 月 14 日